awful.tag: fix and improve tag history
This fixes a long standing tag history breakage. To store history
of tag switching we rely on a special signal "tag::history::update"
which needs to be emitted by any function which deals with tag
selection.
History is multi-level with a configurable limit:
awful.tag.history.limit = 20 (by default).
awful.tag.history.restore function gets a new argument 'idx' which can
be either 'previous' (default) which will revert to the previously
selected set of tags, or a numerical index in the tag history table.
Signed-off-by: koniu <gkusnierz@gmail.com>
Signed-off-by: Julien Danjou <julien@danjou.info>